1. Technical Field
This invention relates to the transmission of data and, in particular, to the transmission of data over voiceband telephone channels using modems.
2. Description of the Prior Art
Voiceband telephone channels have a limited or constrained bandwidth and are thus limited in the rate at which data may be transmitted there over. Depending on the length of the line comprising a voiceband channel and the circuits used to switch such channel, the spectral characteristics of each bandwidth constrained channel may differ with each modem connection established thereover. To obtain a satisfactory data transfer, therefore, modems have traditionally been designed to consider the worst case line for a particular data rate to insure reasonably accurate data transfer at the selected rate.
A number of advances have been achieved in allowing analog modems to more efficiently transmit digital data over voiceband channels. One recent advance in the design of analog modems is provided in U.S. Pat. No. 4,771,232 issued on Sep. 13, 1988, which is hereby incorporated by reference. In this patent, a system and method are disclosed for extracting spectral data from the actual received modem signal that has been shaped by the amplitude characteristics of the line. This extraction of data is achieved without interrupting or affecting the data traffic through the modem and without the use of test tones.
Another recent advance in the design of analog modems is provided in U.S. Pat. No. 5,008,903 issued on Apr. 16, 1991, which is hereby incorporated by reference. In this patent, an apparatus and method are disclosed for calculating preemphasis coefficients for a transmitting modem based upon the noise spectrum as it is received at the receiving modem. A noise spectrum analysis circuit calculates a difference between the transmitted data and the actually received data. This difference is used to compute a discrete Fourier transform of the noise spectrum, which is transmitted back to the transmitting modem on the secondary channel. This data is then used to calculate new pre-emphasis coefficients for use in subsequent transmissions by the transmitting modem.
Other recent design advances are set forth in U.S. Pat. No. 5,103,227 issued Apr. 7, 1992 and U.S. Pat. No. 5,230,010, issued Jul. 20, 1993, both of which are hereby incorporated by reference. U.S. Pat. No. 5,103,227 proposes a fractional rate encoder which provides modulus conversion of an incoming data stream into an output stream having frames of a particular structure. U.S. Pat. No. 5,230,010, issued Jul. 20, 1993 proposes achieving fractional rate modulation by separating incoming data into frames of bits with each frame being partitioned into bit words of unequal lengths.
Although these advances improve the manner in which analog modems transmit digital data over voiceband channels, none of this art identifies the optimum boundary conditions then existing on a bandwidth constrained channel for adapting parameters for maximizing the transmission of data over the channel.